"Raindrops vary in size from about 0.02 in. (0.5 mm) to as much as 0.33 in. (8 mm) in thunderstorms."

0.5–8 mm

Raindrops are common occurrences that everyone has experienced. However, not
many people actually know the real size of a raindrop and the scientific principles
behind this simple tiny glob of matter. A raindrop occurs when the water vapor
from a cloud wraps itself around tiny particles during condensation (the process
by which a gas turns into a liquid). Contrary to popular opinion, raindrops are
not shaped like teardrops. In fact, they are actually oblate spheroids, or spheres
with the nose smashed in.

Different sources approximate different ranges for the measure of a diameter
of a raindrop. However, on the average, a raindrop is between 0.1 to 5 millimeters.
There are some exceptions; rarely, raindrops of 8 millimeters were known
to occur. Sizes larger than that do not normally occur because the raindrop particles
simply break up or collide with other neighboring particles. The appearance of
large raindrops always signals strong updrafts and turbulence.

Presently, precipitation is believed to be triggered by a course of action
called the collision-coalescence process. Most cloud droplets are extremely small
that the motion of the air keeps them suspended. Because large cloud droplets
fall much faster than smaller droplets, they are able to sweep up the smaller
ones in their path and thus grow and expand in size and volume.

Several important factors affect the diameter or size of a raindrop. First,
the fall velocity of a raindrop particle is directly proportional to its diameter.
The larger the particle, the faster it falls. The same follows for the maximum
fall distance before evaporation, or the process in which a liquid turns into
a gas. The larger the diameter, the greater the distance it will fall due to gravity,
the force that pulls a water droplet toward the earth's surface.

As a droplet falls, it also encounters air resistance or frictional force.
The magnitude of this force depends on the size of the drop's "bottom",
or the surface area resisting the fall. Frictional drag increases as the particle
accelerates, or speeds up. The frictional and gravitational forces finally balance
and the droplet falls at a constant speed, called the terminal velocity. This
again depends on size; smaller droplets have a lower terminal velocity than larger
droplets.

"Since Lenard found no drops with diameters less than 0.5 mm (0.02 inch), he did conclude that the updrafts in the clouds must be of sufficient strength to prevent such small drops from falling out. We also know that he only recorded one drop in the 4.75 to 5.25 mm diameter range."

0.5 mm(min.)4.75–5.25(max.)

"By suspending drops of known size, he determined that small drops up to about 2 mm (0.08 inches) in diameter "fell" as spheres. Larger drops, however, deformed while falling acquiring a shape with a flat bottom and rounded top similar to that of a hamburger bun. Thus, Lenard was the first to report that raindrops were not the stereotypical teardrop shape but were spherical when small and shaped much like a hamburger bun when larger."

< 2 mm(spherical)> 2 mm(bun-like)

"Drops, however, became unstable at diameters greater than 5.5 mm (0.21 inches), Lenard found. They lasted less than a few seconds before breaking apart in the airflow, torn asunder by the aerodynamic forces acting on the drop. This observation combined with the lack of drops larger than this size in his rainfall measurements led Lenard to conclude that the maximum drop size possible in nature was just larger than 5 mm."